Acoustic apparatus



May 29, 1923.L 1,456,538

l. B. CRANDALL AcoUsTIc APPARATUS Filed Dec. 24, 1917 4000 8000 12000- /5000 F/z/Mry Il ff w p14" 48 50 4;*7 42 K. M1 49 4 f2 4f 46 Patented May 29, 1923.

UNITED STATES PATENT OFFICE.

IRVING B. CRANDALL, OF NEW YORK, N. Y., ASSIGNOR TO WESTERN ELECTRIC COM- P ANY, INCORPORATED, OF NEW YORK, N. Y., CORPORATION OF NEW YORK.

ACOUSTIC APPARATUS.

Application filed December 24, 1917. Serial No. 208,661.

To all 'tc/tom 'it may concern Be it known that I, InvrNG B. CRANDALL, a citizen of the United States, residing at New York, in the county of N ew York and State of New York, have invented certain new and useful Improvements in Acoustic Apparatus, of which the following is a full, clear, concise, and exact description.

This invention has reference to improvements in acoustic apparatus for the transmission, reproduction, recording, analyzing and measuring of sound waves. More particularly it .relates to 'telephone transmitters` one of its objects being to produce a transmitter' that gives practically a uniform transmission over a greater range of frequency than has heretofore been known.

it has been found that diaphragme suc-h as are used in telephones, phonographs and other acoustic apparatus have a more or less marked natural period for one or more frequencies and if acted upon by waves of these frequencies will respond more than for materially different periodicities. The natural frequency| depends in general on the mass, and stidness of the diaphragm, decreasing with increase in mass but increasing with increase of stiffness.

In the usual telephone diaphragm the lowest natural frequency is in the neighborhood of 1000 cycles per second, which is well within the range of those frequencies used in speech. Because ofv this natura-l periodicity and the resulting inequality in response to impulses of different frequencies, a telephone diaphragm will distort speech waves which are impressed on it when the resonance point of the diaphragm falls within that range which is of importance in speech. lf. however, the elasticity or the stiness of the transmitter is increased sufficiently, this resonance point will be removed outside the speech range and there will be a substantial equality of response 'to all impressed frequencies of the same intensity over a wide range of frequency. This maximum response at the resonance point could also be considerably decreased in value by increasing the damping factor of the vibrating` system. The damping factor, in general, depends only upon the frictional forces and uponv the inertia, being directly proportional to the frictional forces and inversely proportional to the inertia or mass of the moving system. Increasing the damping factor therefor gives a vibrating system a frequency characteristic of response more nearly of the ideal form which would be represented by a line parallel to the axis of frequency, indicating thereby an equal response to all frequencies.

, Obviously the quality of speech as transmitted by telephonie or phonographic apparatus would be considerably improved by increasing the elasticity and the damping factor of the vibrating system. One way this may be accomplished constitutes the subject-matter of a copending application by E. C. Wente, led'December 20, 1916, Serial No. 137,978, now Patent No. 1,333,744, in which a condenser transmitter is described, which consists of a stretched dia-- phragm and a stationary member called a back plate in very close face-to-face relation with each other. As described therein, the close adjustment of the two members to each other by virtue of the thin air film so obtained greatly increases the elasticity of the diaphragm and its damping coeicient, so that the naturaly period peak is removed beyond the upper limit of the frequency range of importance to speech, and reduced to a small value.

It has been found, however, that while such a. system'ofi'ers approximately a uniform response for a wide range, that its response for waves of low frequency depends to some extent upon the frequency. F or example.y in an instrument in which the back plate was a disc of radius 1.63 cm., and separated from the diaphragm by a distance 0.00022 cm., the response falls in value very rapidly from Zero impressed frequency to a frequency of 500 cycles per sec., the response at 500 cycles being less than half that for a constant current voltage of zero frequency. From 500 cycles on to a very high frequency, the response is substantially constant. This distortion at low frequencies is due primarily to the fact that the air between the diaphragm and the adjacent back plate, while escaping freely for low frequency vibrations of the diaphragm, is practically imprisoned for high frequencies, so that the stiffness and damping of the vibrating system is therefore a function of the impressed frequency.

In accordance with this invention, this tween the two members th at would be otherwise completely imprisoned. As one specific embodiment of this invention, this may be accomplished by providing channels or groovesof suitable depth, width andspacing in the back plate of the vibrating system, in order that. the air may escape pre1 crly for high-frequency vibrations of the diaphragm as well as for lower frequencies. Obviously, the same result could also be accomplished by inserting apertures suitably spaced and of proper size in the bach plate.

Itis to he understood that a definite stiffness is obtained from the air film between the sections of the hac-li plate and the dia-` phragm, and that this stiffness is to be added to the inherent stiffness of the diaphragm, in Aorder to obtain the total stiffness at any given frequency'.

It isfalso to be understood that wit-hin ccrtain limits, yarious combinzitions of stiff ness and damping are added to the system in this' invention, the general relation be" tween such.l added stiffness and damping being that when a maximum effect of stiffi'iess is obtained, a minimum of damping is obtained, and vice versa. Thus, to obtain uniform response over a broad range' of frequency, including zero, with a device having a high natural frequency, it is necessary to add both damping and stiffness in definite amounts to the device, by means of the air film between the diaphragm and plate.

However it is within the spirit of this inven tion to so vary the escape of air from the film by the disposition of the grooves or other channels of escape, that all possible combinations of added stiffness and adder'l (lampi g are obtained. For example, as the escape of air is more and more facili.- t'ated', the damping effect ismade preponderant because theincreased air motion allows' more work tohe done against the friction'al forces, while preventing the escape of the airwouldtend to increase only the stiffness. If it is desired to use a device having its ,naturalfrequency peak withinthe' range of ii'nport'ance in speech, it is possible to so apply the' groovesto thje plate member, andY 'regulate its separation from the diaphragm, that the'v stiffness and consequently the naturalvfrequency is inappreciably ehanp'vd, anda maximum of damping is obtained, thereby reducing the peak to a minimum.

This invention will be bet er: understood by reference to the following detailed description, taken in connection with the accompanying drawings, in which Fig. l shows a relation between the amplitude of forced oscillations transmitted, and the frequency of the impressed waves for an air damped transl'nitter; F 2 is a cross-sectional view of a transmitter of one type that this invention may possess; Fig. 8 is a view in lvierspective of al portion of Fig. 2; Fig. il; illustrates this invention in connection with a microphone trai'ismitter; AFig. 5 is a. modification that may be made in the structure of lfig. Fig. f3 is a view in plan of a portion of Fig. 5; and Fig. 7 represents an application of this invention to the reproduction or recording of sound waves.

In `Fig. l., curve A is the characteristic curve for a type of a vibrating' system in which the .stiffness of the vibating system is much greater for high frequency than for low. The ordinates represent the intensity of response and the abscissze represent various values of frequency that sound waves may have that are impressed with equal intensity at all frequencies. It is to be noted that the response for zero frequency is large, but that with increase of frequency there is a qnicl; drop to a minimum value,

where it remains until the resonance pealr begins. rliis drop in rcsllionsiveness, which occurs in the specific instance cited in the neighborhood of 300 to 500 cycles, is caused by the sudden increase in stiffness of the vibrating system, as well as the exceedingly high value of damping at low-frequencies, both of which effects are due to the action of the air between the diaphragm and the plate of the vibrating system, That the air between the diaphragm and plate escapes readily for frequencies in the neighborhood of zero cycles, but as the frequency of the vibration of th diaphragm increases, the eslape of the air from the filmA becomes much more di iiicult, thereby increasing the stiffness of the system and cutting down the intensity of the response until at about 50i) cycles, where in the case we 'are considering, the air seems to be imprisoned practically as much as for considerably higher frequency.

If now, in accordance with this invention. we provide means for allowing the air to escape more readily at high frequencies than for low, the stiffness does not increase so rapidly with frequency, and the decrease in the amount of response, as noted above, would he considerably diminished. A vibrating system of this type properly provided with such a means would therefore have such a characteristic curve of response as that of curve B of Fig. l.

In Fig. 2 is illustrated a cross-sectional view of one form that this invention may possess, which the diaphragm 7 andthe grooved plate S constitute the two members Illu of the vibrating system.. Sound waves incident on the diaphragm cause it to vibrate, thereby causing variations in the thickness of the dielectric in between the plates, and consequently `ive rise to the variations in the capacity of the condenser. The diaphragm i is shown clamped near its ou' eredges by meansv of the clamping` rings 9 and l0. Ring` 12 has in contact with the diaphragm a projection i3 'that serves to stretch the diaphragm when suitable conipressien is inserted between clamping` rings 9 and l0, and the ring l2 by means of the bolt ld. The back electrode or plate 8 is fastened to an' insulating` disc l5 of suitable insulation material, such as balrelite or glass. rlhis disc rests in a recess i6 of 'the bushing` li", in such a manner that when the rim of the bushing` is in contact with the diaphragm near its outer edge, the plate is separated from the diaphragm by avery thin film of air having a thickness of the order from 0.0005 to 0.0020 of an inch. ln order to have a uniform air space of such dimensions it is necessary to have the plate and the rim of the bushing very accurately machined and polished. The diaphragm be stretched by any of the methods known in the art. 18 is a locking ring for the insulating disc and the bushing. 19 is the locking ring for the bushinpY and the ringv l2.

3 is a topview of the plate 8 showing how it may be divided by the grooves 20 into a number of sections 22.

Such an arrangement as that of Fig. 2 will have such a characteristic curve as that of curve B ofV Fig. l, providing proper values of separation and the site of back plate are selected, since the grooves 20 are so positioned that theiilm of air between the diaphragm and the various sections may escape to a certain degree even for relatively high frequencies. The sir/.e of the sections into which a. plate should be divided to obtain the desired result and still preserve a high natural period for the system, depends for vibrating system of given mass largely upon the distance between the diaphragm and the plate, the smaller'the distance between the diaphragm and plate, the smaller should be the area of the sections and the larger the `grooves in between. rllhus, in one specic .illustration with a definite inherent stiffness due to tension in the diaphragm, it was found that the arca of each section, in whicl'i each section approximated a disc, should be amiroxiniately 0.9 sq. cm. in general it may be said that the sire of the in( idual sections of the back plate depends also upon the stiffness per se of the vibrating system. the greater the original stiffness, the larger the `area of the sections may' be to obtain the substantial equality of response desired. It is' true with practically any size disc within reasonable limits that it be placed far enough away from the diaphragm in a condenser transmitted that the air can escape to any desired extentfrom between them and so maintain a fairly uniform response to various frequencies impressed on the diaphragm. However, such an instrument would be less e'ifficient as a transmitter and in addition would possess a hieher internal impedance as a condenser, both of which properties are undesirable. ln general the size of diaphragm and plate that are used are arbitrary, that is, are fixed by other conditions than the requirement of equal ef'hciency over a certain range of frequency. And this invention provides a means for modifying such a plate so that the desired frequency characteristics are obtained.

Fig. e illustrates this invention in connection with a microphone transmitter of similar structure to that of F ig. 2 with the exception that the back plate S has been replaced by back plate 25 in which the central disc contains a carbon chamber 2T. rl`he carbon granules 28 are prevented from clogging the space between the diaphragm and the plate by a thin washer 29. rllhe function of this system is similar to that ,neviously described, since the grooves v20 provide ready means for the escape of the air between the disc and the diaphragm for a broad range of frequencies impressed on the diaphragm 7.

Fig. 5 represents a modification that may be made in the back plate of the vibrating system of Fig. l. ln this modification a back plate, y29. is shown havingl a plurality of apertures 30 employed instead of grooves for providingl low impedance paths to the' outer air for the escape of. the air that would otherwise be imprisoned betr-.Teen the diaphragm andv plate. Obviously the grooves in the plate 25 of Fig. 1-, may be replaced by apertures in a similar manner.

Fig. 6 is a view in plan of the plate 29 of' Fig. 5 and shows a top view of the apertures 30.

'.7 shows how this invention may be adapted to be used as an instrument for recording` or measuring sound waves. l0 is a condenser transmitter which may be of the type described which is shown in either Figs. 2 or 4i. Connected to one terminal of the condenser is a heated filament /ll of a thermionic vamplifier To the other terminal of the condenser is connected the grid element 4:3 of the amplifier, his amplifier beingthe usual thern'xionic amplifier of the audion type now well known in the art. Its output circuit contains the primary transformer winding d5. The secondary winding d6 may be included in a suitable receiving circuit for the amplified electric oscillations from the amplifier which are proportional in intensity and magnitude to the sound 'diaphragm is vibrating. The input circuit has a source oi voltage 50. Vlilith such an arrangement, sound vi v incidei'it upon tbc condenser transmitter (l `would produce cor responding` electric oscillations in the input circuit oi the annalilier 42 which would ap pear in amplified liorm in its output circuit 52 where they could be eitherl passed on to another amplifier Yfor 'further anqiiliiratitm or connected to a systen'l containing; suitable receiving or recordingdevice.

The arrangement oit the cireu scribed does not form part ot th inveution but is described and claimed ii a copending application lilartley, Serial No. 53,429, filed September 2li), 1915, Although this invention is shown and described as a. telephone transmitter, it is to be mideistooil that it may serve equally well as a recei".er and although it has been shown in only two modifications, it may be used equally well in an electromagnetic receii er or transmitter or any other type yof receiver' or transmitter by such obvious changes will occur to anyone skilled in the art.

What is claimed is:

l. A device 'for the transmission ot sound composed oli two members, a gaseous medium therebetween, said members `beingin such a close laccio-.tace relation to each other that the lowestI natural Li'equeigicy of vibration of said device is beyoinl the ran Y ot importance to speech, and means iior p viding low impedance paths 'for the gras i.; prisoned between said members.

2. A. device for the transmis ion ot souinl composed ot two members, a gaseous medium therebetween, said men'iliiers bein in such a close iaceto-'Facc relation to each other that the lowest natural 'frequency ol.t vibration of said device beyond the range of importance to speech, and means comprising `grooves in one of said members whereby low impedance paths are provided :tor the imprisoned between said members.

3. A device For the transmission ot sound comprising a diaphragm and a plate, a gaseous medium therebetween, said diaphragm and plate being in such a close 'taceto-face relation with cach other that the lowest natural 'frequency oi said diaphragm bers being a diaphragm and the other being; a plate in .frisse tarato-tace'relation but out el? contact with said diapln in, when said diaphragm is 'vil rating, plate being grooved on its `tace adjacent to said diaphragm.

5. The device ,tor the transmission oit T. ln a microphone transmittf-n', a vibrate ing' member, and a plate l'faseous medium therebetween, said member and plate being in suoli a close face-to-lacerelation that the lowest natural frequency of said diaphragm is beyond the range of importance in speech, said plate containing a chamber 'for carbon granules, the remainder of said plate being divided into a plurality oil sections by grooves.

8. A device iter the transmission ot sound, con'iposed of two members, a gase us medium therebetween, said members being;` in such a close face to `face relation to each other that the lowest natural frequency ot vibration of said device is beyond the range of importance to speech, and means for allowing the restricted escape oi the pas imprisoned between said members `while still maintaining said hin'h natural frequency,

9. A telephone device .comprising a vibratingi` member, the normal natural tre quency o'lf which is within the range ol speech frequency, and means for increasing the natural 'frequency of said vibrating member above the range Of essential speech 'l'requelicies andv for making said member substantially uniformly responsive to 'frequenciesv ,trom zero to the order of 5002 cycles per secondn l0. A microphone transmitter compris ing; a vibratingl member and a plate, a gaseous medium therebetween, said member and plate being in such close face to :tace relation to each other that the stiffness ot the system for impressed 'frequencies is in-l substantially uniform the responseof said transmitter for frequencies within the range of importance to speech.

ll. A microphone transmitter comprising a diaphragm, and a plate presenting a discontinuous surface to said diaphragm, said diaphragm and said plate being in such close face-to-face relation that the stiffness of -the system is increased appreciably by said rclation, said transmitter being provided with a chamber for granular materia-i which cooperates with said diaphragm.

l2. A microphone transmitter comprising a diaphragm, a plate presenting a discontinuous surface to said diaphragm, said diaphragm andsaid plate being in such close face-to-face relation Vthat the stiifness of the system is increased appreciably by said relation, and granular material contacting with a portion of said diaphragm.V

13. A microphone transmitter comprising` a diaphragm, a plate presenting a discontinuous surface to said diaphragm, said diaphragm and said plate being in such close face-to-face relation that the stidness of the system is increased appreciably by said relation, granular material cooperating with said diaphragm, and means for holding said granular. material in its desired position with respect t0 saiddiaphragm.-

14C. A microphone transmitter comprising a vibrating member, a plate having a discontinuous surface in such close face-to-face relation therewith as to appreciably retard the escape of the gas therebetween, said transmitter being provided with a chamber for granular material on the same side of said member as said plate, and means for causing the granular material to contact with only a restricted portion of said vibrating member.

15. A device for the transmission of sound comprising a diaphragm, and means comprising a plate in close face-to-face relation with said diaphragm for making said dia.- phragm have a substantially constant response te impressed frequencies of less than l,000 cycles per second, said plate presenting discontinuous surface to said diaphragm and saidl plate being in such close face-torace relation with said diaphragm that the enclosed film of gas between said diaphragm and said plate escapes readily for low frequencies, but is appreciably retarded by friction for the higher frequencies of import-ance in speech.

16. A device for the transmission of sound comprising a stretched diaphragm, a plate in close face-to-face relation with said diaphragm, a gaseous medium between said plate and said diaphragm, said plate having discontinuities symmetrically disposed of its face adjacent said diaphragm for allowing the gas enclosed therebetween to escape at such a restricted rate that the device has a substantially constant response for impressed frequencies from zero to 6,000 cycles per second.

17. A device for the transmission of sound comprising a stretched diaphragm, a plate in close face-toface relation with said diaphragm, a gaseous medium between said diaphragm and said plate, said plate having discontinuities symmetrically arranged with respect to the center of said plate on the side of said plate adjacent said diaphragm for causing said device to have a substantially constant response over a wide range of frequencies of importance in speech,

ln witness whereof, l hereunto subscribe myzname this 22nd day of December, A. D. 191

IRVING B. CRANDALL. 

